Mechanism of Dye Degradation during Electrochemical Treatment

Studies were conducted for understanding the electrochemical (EC) degradation mechanism of a triphenylmethane dye, namely, basic green 4 (BG), commonly known as malachite green with aluminum electrode. At the optimum conditions (current density = 117.64 A m–2, initial dye concentration = 125 mg L–1, pH = 6.5, electrode gap = 1 cm, and NaCl concentration = 1.5 g L–1), more than 85% BG degradation was observed within 50 min of treatment. UV–visible and Fourier transform infrared (FTIR) spectroscopy, high performance liquid chromatography (HPLC), gas chromatography–mass spectroscopy (GCMS), and high-resolution mass spectroscopy (HRMS) analysis showed that the degradation occurred via the cleavage of conjugated structure and N-demethylation. The intermediate products identified included hydroxymethylated intermediates during the N-demethylation of the dye; and N,N,N′,N′-tetramethyl-4,4′-diaminobenzophenone, 4,4′-bisaminobenzophenone and N-methyl-para-aminophenol after cleavage of the conjugated triphenylmethane ring. Zeta potential study indicated a hard acid–base interaction between aluminum ions and hydroxides generated in situ during the EC treatment process and the −N(CH3)2 group of dye molecules. Generation of active species such as hydrogen peroxide, ozone, and chlorinated oxidizing compounds was observed during the EC treatment process and that the BG degradation occurred via a •OH radical attack.